While the weight of matter is entirely dependent upon the strength of local gravity, the mass of matter is constant (assuming it is not traveling at a relativistic speed with respect to an observer). Accordingly, for astronauts in microgravity, no effort is required to hold objects off the cabin floor since such objects naturally hover; they are “weightless.” However, since objects in microgravity still retain their mass, an astronaut must exert ten times as much force to accelerate a 10-kilogram object at the same rate as a 1-kilogram object.

Overview

Mass is an inertial property. Inertia is sensed when a bowling ball is pushed horizontally on a level, smooth surface. This is quite distinct from “weight,” which is the downwards gravitational force of the bowling ball that one must counter when holding it off the floor. Unless relativistic effects apply, mass is an unchanging, universal property of matter that is unaffected by gravity. Weight on the other hand, is a property of matter that is entirely dependent upon the local strength of gravity. For instance, an astronaut’s weight on the Moon is one-sixth of that on the Earth, whereas his mass has changed little during the trip. Consequently, wherever the physics of recoil kinetics (mass, velocity, inertia, inelastic and elastic collisions) dominate and the influence of gravity is a negligible factor, the behavior of objects remains consistent even where gravity is relatively weak. For instance, billiard balls on a billiards table would scatter and recoil with the same speeds and energies after a break shot on the Moon as on Earth; they would however, drop into the pockets much more slowly.

In the physical sciences, the terms “mass” and “weight” are rigidly defined as separate measures in order to enforce clarity and precision. In everyday use, given that all masses on Earth have weight and this relationship is usually highly proportional, “weight” often serves to describe both properties, its meaning being dependent upon context. For example, in commerce, the “net weight” of retail products actually refers to mass and is properly expressed in pounds (U.S.) or kilograms (see also Pound: Use in commerce). Conversely, the “load index” rating on automobile tires, which specifies the maximum structural load for a tire in kilograms, refers to weight; that is, the force due to gravity. Before the late twentieth century, this distinction was not as strictly applied, even in technical writing, so that expressions such as “molecular weight” (for molecular mass) are still seen.

Converting units of mass to equivalent forces on Earth

When an object’s weight (its gravitational force) is expressed in kilograms, the unit of measure is not a true kilogram; it is the kilogram-force (kgf or kg-f), also known as the kilopond (kp), which is a non-SI unit of force. All objects on Earth are subject to a gravitational acceleration of approximately 9.8 m/s2. The CGPM (also known as the “General Conference on Weights and Measures”) fixed the value of standard gravity at precisely 9.80665 m/s2 so that disciplines such as metrology would have a standard value for converting units of defined mass into defined forces and pressures. In fact, the kilogram-force is defined as precisely 9.80665 newtons. As a practical matter, gravitational acceleration (symbol: g) varies slightly with latitude, elevation and subsurface density; these variations are typically only a few tenths of a percent. See also Gravimetry.

Professionals in engineering and scientific disciplines involving accelerations and kinetic energies rigorously maintain the distinctions between mass, force, and weight, as well as their respective units of measure. Engineers in disciplines involving weightloading (force on a structure due to gravity), such as structural engineering, first convert loads due to objects like concrete and automobiles—which are always tallied in kilograms—to newtons before continuing with their calculations. Primarily, this is because material properties like elastic modulus are measured and published in terms of the newton and pascal (a unit of pressure derived from the newton). For all practical engineering purposes on Earth, mass in kilograms is converted to weight in newtons by multiplying by 9.80665 (standard gravity).

Buoyancy and “conventional mass”

The masses of objects are relatively invariant whereas their weights vary slightly with changes in barometric pressure, such as with changes in weather and altitude. This is because objects have volume and therefore have a buoyant effect in air. Buoyancy—a force that opposes gravity—reduces the weight of all objects immersed in fluids. This means that objects with precisely the same mass but with different densities displace different volumes and therefore have different buoyancies and weights.

Normally, the effect of air buoyancy is too small to be of any consequence in normal day-to-day activities. For instance, buoyancy’s diminishing effect upon one’s body weight (a relatively low-density object) is 1/860 that of gravity and variations in barometric pressure rarely affect one’s weight more than ±1 part in 30,000. In metrology however, mass standards are calibrated with extreme accuracy, so air density must be taken into account to allow for buoyancy effects.

Given the extremely high cost of platinum-iridium mass standards like the International Prototype Kilogram (IPK), high-quality “working” standards are made of special stainless steel alloys that occupy greater volume than those made of platinum-iridium, which have a density of about 21,550 kg/m3. For convenience, a standard value of buoyancy relative to stainless steel was developed for metrology work and this results in the term “conventional mass.” Conventional mass is defined as follows: “For a mass at 20 °C, ‘conventional mass’ is the mass of a reference standard of density 8000 kg/m3 which it balances in air with a density of 1.2 kg/m3.” The effect is a small one, 150 ppm for stainless steel mass standards, but the appropriate corrections are made during the calibration of all precision mass standards so that they have the true mass indicated on them.

In routine laboratory use, the reading on a precision scale when a stainless steel standard is placed upon it is actually its conventional mass; that is, its true mass minus buoyancy. Also, any object compared to a stainless steel mass standard has its conventional mass measured; that is, its true mass minus an unknown degree of buoyancy. For certain high-precision disciplines, the density of a sample is sometimes known or can be closely estimated (such as when weighing aqueous solutions) and the effect of buoyancy is compensated for mathematically.

Types of scales and what they measure

Technically, whenever someone stands on a balance-beam-type scale at a doctor’s office, they are truly having their mass measured. This is because balances (“dual-pan” mass comparators) compare the weight of the mass on the platform with that of the sliding counterweights on the beams; gravity serves only as the force-generating mechanism that allows the needle to diverge from the “balanced” (null) point. Balances can be be moved from Earth’s equator to the poles without spuriously indicating that objects became over 0.3% more massive; they are immune to the gravity-countering centrifugal force due to Earth’s rotation about its axis. Conversely, whenever someone steps onto spring-based or digital load cell-based scales (single-pan devices), they are technically having their weight (force due to strength of gravity) measured. On force-measuring instruments such as these, variations in the strength of gravity affect the reading. As a practical matter, when force-measuring scales are used in commerce or hospitals, they are calibrated on-site and certified on that basis so the measure is mass, expressed in kilograms, to the desired level of accuracy.